In 1902, Wolff proposed a mechanism to explain the formation of carboxylic acids from diazo‐oxides; Süs in 1944 invoked this same mechanism for the formation of indene carboxylic acid from diazo‐oxides. Since these materials are currently used in the well known class of AZ photoresists, the Süs mechanism has been referred to many times to explain the positive working nature of the photoresists. In spite of the widespread use and industrial importance of the AZ resists, the validity of the Süs mechanism had not been unequivocally demonstrated to any degree of satisfaction. For these reasons, photochemical studies were initiated to establish the following: (1) Obtain direct spectroscopic evidence for ketene intermediates in the Wolff rearrangement of diazooxides. (2) Determine the reactivity of the ketene towards water and hydroxyl containing organic compounds. (3) Determine the reactivity of the ketene in the AZ resists. Fundamental photochemical decomposition mechanisms have been established for the AZ‐type photoresists; these and recent theoretical results on the Wolff rearrangement are used to convey the technological implications of the studies. Copyright © 1980 Society of Plastics Engineers, Inc.